Figure 4.Upper panels: Scalp
distributions of the differences for depressed individuals (left) and
for healthy controls (right) are depicted. Lower panels: Grand-average
ERP waveforms were pooled across electrode sites Pz, Cz, CPz, C1, and C2
during imagery for depression (left) and control (right).
4.Discussion
In this study, we investigated the neural response to emotional imagery
in depressed individuals and healthy controls when they imagine
different emotional images using an encoding-imagery paradigm. The
behavioral results showed that depressed individuals scored lower on
valence ratings for sad and neutral imagery compared to healthy
controls. Furthermore, the ERP findings revealed that
depressed participants
exhibited
enhanced LPP responses to sad imagery relative to happy
imagery, whereas healthy controls
showed greater LPP response to happy imagery relative to sad
imagery. Notably, depressed
individuals exhibited enhanced LPP responses to sad imagery compared to
healthy controls in both middle and late time windows, but not during
the early time window.
Our findings indicate that there are
differential LPP responses to emotional imagery over centroparietal
sites between depressed individuals and healthy controls, which are
broadly consistent with previously observed LPP responses in depression
(Auerbach et al., 2015; Dainer-Best
et al., 2017; Speed et al., 2020). Specifically, depressed individuals
exhibited enhanced LPP responses to sad imagery relative to happy
imagery, whereas healthy controls
demonstrated the opposite pattern.
Namely,
depressed individuals exhibit similar pattern of neural responses to sad
imagery as they do to negative autobiographical memories, and
self-referenced negative words. These findings may be related to the
sustained attentional engagement to negative stimuli in depressed
individuals (Auerbach et al., 2015; Dainer-Best et al., 2017), and may
be partially explained by the potent emotional impact of imagery and its
close association with emotional
memory (Holmes et al., 2009, 2016).
Notably, no significant difference was found in LPP responses between
emotional (happy and sad) and neutral imagery, which probably because
the LPP response to neutral images containing people were comparable to
emotional images (Ferri et al., 2012; Weinberg & Hajcak, 2010).
Critically, depressed participants showed larger LPP responses to sad
imagery compared to healthy controls, especially during the middle and
late time windows. These findings support the presence of mood-congruent
imagery biases in depressed individuals (LeMoult & Gotlib, 2019).
According to Beck’s cognitive model of depression, individuals with
depression have mood-congruent schemas that lead depressed individuals
to exhibit negative information-processing biases
and distort the processing of
emotional stimuli, leading to enhanced reactivity (Beck, 1967).
Additionally, the negative bias for sad imagery in depressed individuals
was mainly observed during the middle and late time windows, suggesting
that depression-related abnormalities in emotional processing primarily
occur more during the top-down attentional capture phase rather than
automatic attentional capture phase of emotional information. Namely,
depression risk is associated with later and more elaborate processing
of negative emotional information (Speed et al., 2016). Benau et al.
(2019) also reported group differences in LPP were specific to the
middle time window. The possible reason is that the earlier LPP reflects
automatic attentional capture of salient information, and the later LPP
is more influenced by top-down attentional capture (Hajcak et al., 2010;
Olofsson et al., 2008). In summary, our findings indicate that sad
imagery captures more cognitive resources during later stages of
information processing in depressed individuals.
However, our results contrary to the
study conducted by Bauer and MacNamara (2021), where they observed a
blunted LPP to negative imagery in depressed individuals.
These discrepancies in the LPP to
valence may be attributed to variations in factors such as age of
depression onset, participant characteristics, stimulus properties, and
task types (Benau et al., 2019; Grunewald et al., 2019; Weinberg et al.,
2016). Bauer and MacNamara’s (2021) study involved participants with
complex internalizing psychopathology and utilized general negative
imagery as stimuli, which may have influenced the neural response.
Depressed individuals process
negative stimuli, particularly sad ones, better and more accurately
(Delle-Vigne et al., 2014). In our study, we utilized sad and happy
images that contain people as emotional stimuli instead of general
positive (i.e., erotic, food or
flower images) and negative stimuli (i.e., sad or threatening scenes).
Consequently, we did not observe a main effect between depressed
individuals and healthy controls in LPP amplitudes for happy and neutral
imagery, but only for sad imagery. The differences likely reflect
specific processing biases toward sad imagery rather than a broad bias
in emotional information processing. Collectively, our findings suggest
that an enhanced LPP response to sad imagery may serve as a potential
biomarker of depression risk.
In line with the ERP results, depressed individuals rated sad and
neutral imagery more negatively compared to healthy controls, while
their ratings for happy imagery were comparable. Our behavioral results
are consistent with previous studies showing a bias toward endorsing and
recall more negative stimuli in depression (Benau et al., 2019;
Dainer-Best et al., 2017; Speed et al., 2016). The finding provides
behavioral evidence for depressed participants experiencing more
negative imagery (Holmes et al., 2016; Weßlau & Steil, 2014),
supporting the presence of mood-congruent biases in depression. Overall,
the tendency toward a negativity bias in imagery valence ratings
observed in depressed individuals may serve as a potential cognitive
feature associated with vulnerability to depression.
Caution should be exercised when interpreting the present results due to
several limitations. Firstly, the
relatively small sample size limited the interpretability of effect
sizes, and the use of homogeneous college student limited the
generalizability of these findings. Future studies should employ larger
and more diverse populations to enhance statistical power and generalize
to broader populations. Second, our
study compiled a new set of pure emotion materials to ensure idiographic
emotional stimuli for happy, sad, and neutral images. However, these
images were used for the first time in the present study, replication in
future studies is necessary to establish their reliability and validity.
Finally, although our study identified enhanced LPP activity to sad
imagery as a potential risk indicator for depression, it did not include
longitudinal follow-up to determine whether those participants with
enhanced LPP to sad imagery subsequently develop more severe depressive
symptoms.
In summary, this study contributes novel behavioral and
electrophysiological evidence regarding emotional imagery differences
between depressed individuals and healthy controls.
Both behavioral and ERP results
support the presence of a mood-congruent processing bias in depressed
individuals. Additionally, sad
imagery bias is evident in the middle and late time windows, indicating
a greater engagement of top-down attentional resources in depressed
individuals. These findings shed light on the cognitive and neural
processes underlying emotional imagery in depression.
Reference
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